1. Field of the Invention
This invention relates to the application of flux to the surface of molten steel in a container and more particularly to an improved process for the application of flux in a continuous casting tundish.
2. Description of the Prior Art
In a continuous steel casting operation, liquid steel is teemed from a ladle into a tundish where a reservoir of metal is maintained and from which the metal flows at a controlled rate into one or more casting molds. In such an operation, it is the standard practice to provide a flux or slag cover on the molten steel in the tundish, and molten steel flows from the ladle into the tundish through a ladle shroud tube which penetrates below the flux layer.
The tundish flux layer performs three primary functions as explained, for example, in U.S. Pat. Nos. 4,738,719 and 5,366,535. These function are providing a thermal insulating blanket covering the top surface of the molten steel, protecting the steel from exposure to the atmosphere to prevent atmospheric oxidation, and the absorption from the molten steel mass of inclusions which come into contact with the molten flux surface.
Numerous flux compositions are known and used to produce a tundish slag to achieve desired results depending on the composition of the molten steel and, to some extent, the effect of the flux on the refractory of the tundish and/or the ladle shroud. Also, flux composition may be varied to affect the physical characteristics of the slag, including its fluidity, the slag build-up on the tundish wall, and the ease with which the ladle shroud may penetrate the slag layer. Inclusions such as alumina particles are absorbed or scavenged from the liquid steel only into the liquid phase or layer of flux so that relatively rapid fusion and distribution of the flux over the steel surface is important at the beginning of a casting campaign or where a new tundish is put into use. At the same time, if the melting temperature of the flux is too low, the flux will become excessively fluid with extended use of the tundish, resulting in adverse effects on other features such as the thermal insulating quality of the slag layer.
As is known, the steel temperature is not uniform throughout the tundish in a continuous casting operation. For example, the temperature in the vicinity of the ladle shroud, sometimes referred to as the pour box, is higher than in the area near the tundish outlet, or outlets. As a result of this temperature differential, a liquid flux layer is generated relatively quickly in the area of the pour box, even when using a flux with a relatively high melting temperature, whereas as many as four or more heats may be cast before an adequate liquid flux layer is produced in areas remote from the ladle shroud. Also, ladle slag carried over into the tundish is very fluid and contributes to the formation of a liquid slag layer in the pour box area. Reducing the flux melting temperatures to quickly obtain a liquid cover in the area of the tundish outlet can result in the slag in the area of the pour box becoming excessively fluid, thereby reducing its insulating capability. Also, during ladle change-over, the liquid slag can solidify into a glassy matrix, making it difficult to submerge the ladle shroud on the next heat.
It is known to employ a double-layer slag covering consisting of a first basic flux layer in contact with the steel and an insulating layer of powdery material floating on the top of the first layer. This insulting layer generally consists of or includes a highly siliceous (90-95% by weight) rice hull ash, risking contamination and reducing the effectiveness of the first basic flux layer, although the procedure does substantially reduce the problem of the frozen surface of the slag resisting penetration of the ladle shroud.
To accelerate the generation of a liquid flux layer, it has generally been recommended that the flux chemistry be modified by increasing the silica content and/or by adding fluorspar or calcium fluoride to the parent flux. Increasing the silica or acidic oxide content of the tundish flux is a departure from a basic flux practice, however, and increasing the silica tends to simultaneously increase the FeO and MnO content of the slag. Silica, FeO and MnO can reoxidize steel and create alumina inclusions instead of helping to remove existing inclusions. Further, the addition of fluorspar can increase the alumina absorption capacity so as to quickly erode the alumina graphite ladle shroud and tundish refractories. This drawback of fluorspar is evidenced even in the carryover of ladle slag containing fluorspar into the tundish.
Various means are known and used to control or influence the flow of liquid metal through the tundish from the vicinity of the pour box to the outlets. These may include impact pads designed to produce desired currents in the liquid steel, as well as weirs and/or baffle walls projecting upwardly from the tundish floor. Baffle walls extending the full height and width of the tundish are frequently used, and these walls are provided with a pattern of opening or channels extending therethrough in a direction to produce the desired flow pattern to minimize channeling and to enhance contact of the liquid steel with the molten flux layer to promote the pick-up of inclusions from the steel. Examples of such devices may be found, for example, in U.S. Pat. Nos. 5,551,672, 5,169,591 and 4,852,632.
Despite the improvements made in recent years both in fluxing materials and practice and in tundish design, the increasing demand for higher quality steel continues to pressure the industry for further improvements. Accordingly, it is a primary object of the present invention to provide an improved tundish fluxing process in a continuous casting operation to optimize the heat insulation and inclusion absorption function of the flux.
Another object is to provide such a tundish fluxing process which enables a more uniform liquid flux layer to be maintained over the surface of the liquid steel in a continuous casting operation.
Another object is to provide such an improved fluxing process which does not require expensive changes in the tundish design or casting operation.
Another object is to provide such an improved process which utilizes known flux compositions in a manner to more efficiently achieve the fluxing functions described above.
The foregoing and other objects and advantages are achieved in accordance with the present invention in which different flux compositions are employed to provide the slag cover in different areas of the tundish in a continuous casting operation. Specifically, a flux having a higher melting temperature such as a high lime (CaO) low alumina (Al2O3) flux is employed in the pour box area of the tundish, while a flux with a lower melting temperature such as a high lime, high alumina flux is employed in areas of the tundish remote from the pour box. Due to the higher steel temperature in the pour box area and to mixing with any ladle slag carried over into the tundish, a liquid flux cover is normally quickly produced in this area, even when using a relatively low alumina flux having a higher melting temperature. Since a higher alumina flux has a lower melting temperature, it is employed in areas where the steel temperature is slightly lower, thereby also quickly producing a liquid flux cover for the entire tundish. Flow divider means, preferably the tundish baffle(s), separates the two areas and effectively prevents intermixing of the fluxes in the different areas. By maintaining a desired liquid flux layer thickness over the entire tundish area, the efficiency of the flux as a heat insulation cover and as an inclusion absorption medium is optimized.
The use of a relatively low melting flux in the tundish outlet area results in quickly producing an effective slag for absorbing inclusions from the initial heats in a tundish. Since the liquid cover produced by a fast melting flux has reduced heat insulation capabilities, after the initial heats are cast from a tundish, a compatible higher melting temperature flux can be added to form a powder layer on top of the liquid layer in the outlet area of the tundish. By employing a fast melting flux with a higher alumina content in contact with the liquid steel and a lower alumina flux layer on top, an optimum lime-to-alumina ratio for inclusion absorption may be maintained throughout the casting sequence by the continuous interaction of the alumina enriched liquid slag layer and the covering low alumina flux layer which simultaneously enhancing the thermal insulation efficiency of the slag.